What's the difference between the "Run Instant Measurement" button on synthetic monitors management and clicking the lightning bolt to run an instant measurement of a monitor?

The button will take you to a separate screen where you can configure a unique measurement before running that is not linked to any of your current, existing monitors. The lightning bolt next to an individual monitor will run a measurement that is configured identically to your monitor but does not show up alongside the regularly scheduled data. To view instant measurement results, view the Instant Measurement Log at the top right of the synthetic monitoring management page.